Within the scope of mobile communications, considerable efforts have been made in order to use linear modulation techniques to improve spectral and power efficiency.
In particular, one of the biggest challenges in designing modern wireless transceivers consists in the trade-off between linearity and efficiency of RF amplifiers.
On the one hand, in fact, spectrally efficient modulated signals, such as W-CDMA (Wideband Code Division Multiple Access) and OFDM (Orthogonal Frequency-Division Multiplexing) signals, affect the non-linearity of the amplifier, with consequent undesired in-band distortions and out-of-band spectral re-growth. Such distortions therefore involve the violation of the narrow requirements defined by communication standards.
On the other hand, the improvement of the efficiency of the amplifiers results in a significant saving in terms of cooling and running costs.
To this must be added the fact that the use of a simple direct-conversion transmitter is not practical for three main reasons.
First of all, because energy efficiency is one of the necessary requirements of the system, an efficient but non-linear power amplifier must be used. This therefore involves the use of some kind of linearizer so as to reach the spectral efficiency required.
Furthermore, a direct-conversion system is very sensitive to gain imbalance and to phase and DC voltage offset errors of the modulator in quadrature. An accurate control of the errors of the modulator itself therefore becomes necessary.
Finally, both the non-linearity of the power amplifier and the errors of the modulator in quadrature can vary according to the temperature, the channel frequency, the polarization of the device and the degradation of the components. The use is therefore necessary of an instrument for monitoring the power amplifier and the modulator in quadrature.
Consequently, a high-performance direct-conversion transmitter necessarily requires the fitting of a predistortion, a control of the errors of the modulator in quadrature, and the adaptation of both in order to maintain the perfect performance of the system.
Baseband digital predistortion is one of the most effective techniques used to reach greater efficiency in terms of power and in terms of increase of data speed per unit of band width.
The precision and the flexibility of digital predistortion permit using a non-linear amplifier to increase the total energy efficiency in compliance with the stringent performance requirements called for.
Today, the use of digital cellular transmitters with integrated processors has increased the importance of baseband digital predistortion, which benefits from the low manufacturing cost and the high flexibility offered by the design of digital circuits. Consequently, in modern architectures, these algorithms are made in the digital ambit using FPGA (Field Programmable Gate Array) devices adapted to implement digital predistortion algorithms (DPD) and quadrature modulation correction (QMC) algorithms.
As schematically shown by way of example in FIG. 1, the digital predistortion function DPD acts on the data sent for the purpose of cancelling the distortion in the power amplifier PA, implementing a reverse model of the amplifier. The predistortion function is applied to the sequence of sent digital data x(n) and is adapted to model the non-linearity of the amplifier PA.
The estimate of parameters DPD is based on samples of the output y0(n) and of the input z(n) of the power amplifier PA. In particular, to separate the linear effect of the power amplifier PA and of the circuit that pilots it, the estimate is based on the aligned output y(n). The alignment process considers the variations in amplitude, the delay and the phase variations of y0(n) compared to z(n).
As shown in the diagram in FIG. 2, in the case of an amplifier RFin to RFout, the input RF IN signal must be reconverted in IF or in baseband (signal IQ), brought to the function of digital predistortion DPD and then converted again into an RF OUT signal. Similarly, the RF OBS signal is converted into IF or into baseband IQ and is brought to the function of digital predistortion DPD.
The architecture described above can be implemented in different ways and the conversions into IF or baseband IQ can be used with relative advantages and drawbacks.
The conversion into/from baseband IQ in analog ambit is the best way to achieve ample band width, high precision and cost optimization, and the more the band width of the RF signal is ample (i.e., up to 75 MHz or more) the more important this becomes. Furthermore, the output path and the observer path (OBS) must necessarily have a band width five times greater (i.e., up to 375 MHz or more) due to the distortions of the third and fifth order which could be introduced by the PA.
Such technique is not without its drawbacks however inasmuch as it calls for identical paths for the way I and the way Q which cannot be achieved in analog ambit.
The gain imbalance and the phase difference between the I and Q paths involve spurious signals that again fall within the transmission band and which cannot be filtered by means of RF or digital filters.
Furthermore, the offset DC (always present in analog components) and the oscillator of the RF converter involve a spurious frequency that falls again at the center of the transmission band.
Neither of the spurious components are negligible and both considerably affect the correct operation of the amplifier.
Conversion into/from IF permits locating the disturbances outside the IF work band, making it possible to filter such disturbances by means of an IF low pass filter and delegating the IQ conversion to digital domain.
The use of an IF conversion nevertheless requires a wider band (theoretically two times but in practice three to four times the signal band envisaged for the IF antialiasing filter) with consequent high speed of data transmission in the A/D and D/A converters, something that represents a far from negligible problem for the D/A output and for the A/D observer inasmuch as, as has been said, these carry a signal which is five times the band width of the input signal (i.e., up to 375 MHz or more).
The purpose of using an algorithm QMC (Quadrature Modulator Correction) is to change the continuous component, the phase and gain of the signal received or sent, so as to offset the errors found by the modulator in quadrature.
With reference to an ideal case, the algorithm QMC would be able to eliminate the birdies in correspondence to the local oscillator frequency and image frequency.
Nevertheless, the errors relating to the continuous component, to the offset and to the phase can increase according to the temperature and time. Consequently, the application of the algorithm QMC in the ambit of real appliances calls for the use of a feedback system which permits estimating such errors and which is able to pilot the appliance so as to correct the errors dynamically.